FinFET and method of fabricating the same

ABSTRACT

The disclosure relates to a fin field effect transistor (FinFET). An exemplary structure for a FinFET comprises a substrate comprising a major surface; a first fin and a second fin extending upward from the substrate major surface to a first height; an insulation layer comprising a top surface extending upward from the substrate major surface to a second height less than the first height, whereby portions of the fins extend beyond the top surface of the insulation layer; each fin covered by a bulbous epitaxial layer defining an hourglass shaped cavity between adjacent fins, the cavity comprising upper and lower portions, wherein the epitaxial layer bordering the lower portion of the cavity is converted to silicide.

TECHNICAL FIELD

The disclosure relates to integrated circuit fabrication and, more particularly, to a fin field effect transistor (FinFET).

BACKGROUND

As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a fin field effect transistor (FinFET). A typical FinFET is fabricated with a thin vertical “fin” (or fin structure) extending from a substrate formed by, for example, etching away a portion of a silicon layer of the substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over (e.g., wrapping) the fin. Having a gate on both sides of the channel allows gate control of the channel from both sides. In addition, strained materials in source/drain (S/D) portions of the FinFET utilizing selectively grown silicon germanium (SiGe) may be used to enhance carrier mobility.

However, there are challenges to implementation of such features and processes in complementary metal-oxide-semiconductor (CMOS) fabrication. For example, non-uniform distribution of silicide on strained materials causes high contact resistance of source/drain regions of the FinFET, thereby degrading the device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flowchart illustrating a method of fabricating a FinFET according to at least one embodiment of the present disclosure; and

FIGS. 2A-14B are perspective and cross-sectional views of a FinFET at various stages of fabrication according to various embodiments of the present disclosure.

DETAILED DESCRIPTION

It is understood that the following description provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Referring to FIG. 1, illustrated is a flowchart of a method 100 of fabricating a fin field effect transistor (FinFET) according to at least one embodiment of the present disclosure. The method 100 begins with step 102 in which a substrate comprising a major surface is provided. The method 100 continues with step 104 in which a first fin and a second fin are formed extending upward from the substrate major surface to a first height. The method 100 continues with step 106 in which an insulation layer comprising a top surface is formed extending upward from the substrate major surface to a second height less than the first height, whereby portions of the fins extend beyond the top surface of the insulation layer. The method 100 continues with step 108 in which an epitaxial layer is selectively grown covering each fin. The method 100 continues with step 110 in which the substrate is annealed to have each fin covered by a bulbous epitaxial layer defining an hourglass shaped cavity between adjacent fins, the cavity comprising upper and lower portions. The method 100 continues with step 112 in which a metal material is formed over the bulbous epitaxial layer. The method 100 continues with step 114 in which the substrate is annealed to convert the bulbous epitaxial layer bordering the lower portion of the cavity to silicide. The discussion that follows illustrates embodiments of FinFETs that can be fabricated according to the method 100 of FIG. 1.

FIGS. 2A-14B are perspective and cross-sectional views of a fin field effect transistor (FinFET) 200 at various stages of fabrication according to various embodiments of the present disclosure. As employed in the present disclosure, the FinFET 200 refers to any fin-based, multi-gate transistor. The FinFET 200 may be included in a microprocessor, memory cell, and/or other integrated circuit (IC). It is noted that the method of FIG. 1 does not produce a completed FinFET 200. A completed FinFET 200 may be fabricated using complementary metal-oxide-semiconductor (CMOS) technology processing. Accordingly, it is understood that additional processes may be provided before, during, and after the method 100 of FIG. 1, and that some other processes may only be briefly described herein. Also, FIGS. 2A through 14B are simplified for a better understanding of the concepts of the present disclosure. For example, although the figures illustrate the FinFET 200, it is understood the IC may comprise a number of other devices comprising resistors, capacitors, inductors, fuses, etc.

FIG. 2A is a perspective view of the FinFET 200 having a substrate 202 at one of the various stages of fabrication according to an embodiment, and FIG. 2B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 2A. In one embodiment, the substrate 202 comprises a crystalline silicon substrate (e.g., wafer). The substrate 202 may comprise various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). In some embodiments, the doped regions may be doped with p-type or n-type dopants. For example, the doped regions may be doped with p-type dopants, such as boron or BF₂; n-type dopants, such as phosphorus or arsenic; and/or combinations thereof. The doped regions may be configured for an n-type FinFET, or alternatively configured for a p-type FinFET.

In some alternative embodiments, the substrate 202 may be made of some other suitable elemental semiconductor, such as diamond or germanium; a suitable compound semiconductor, such as gallium arsenide, silicon carbide, indium arsenide, or indium phosphide; or a suitable alloy semiconductor, such as silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. Further, the substrate 202 may include an epitaxial layer (epi-layer), may be strained for performance enhancement, and/or may include a silicon-on-insulator (SOI) structure.

The fins are formed by etching into the substrate 202. In one embodiment, a pad layer 204 a and a mask layer 204 b are formed on the semiconductor substrate 202. The pad layer 204 a may be a thin film comprising silicon oxide formed, for example, using a thermal oxidation process. The pad layer 204 a may act as an adhesion layer between the semiconductor substrate 202 and mask layer 204 b. The pad layer 204 a may also act as an etch stop layer for etching the mask layer 204 b. In an embodiment, the mask layer 204 b is formed of silicon nitride, for example, using low-pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). The mask layer 204 b is used as a hard mask during subsequent photolithography processes. A photo-sensitive layer 206 is formed on the mask layer 204 b and is then patterned, forming openings 208 in the photo-sensitive layer 206.

FIG. 3A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 3B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 3A. The mask layer 204 b and pad layer 204 a are etched through openings 208 to expose underlying semiconductor substrate 202. The exposed semiconductor substrate 202 is then etched to form trenches 210 with major surfaces 202 s of the semiconductor substrate 202. Portions of the semiconductor substrate 202 between trenches 210 form semiconductor fins 212. The semiconductor fins 212 comprise a first fin 212 a and a second fin 212 b extending upward from the substrate major surface 202 s to a first height H₁. Trenches 210 may be strips (viewed from in the top of the FinFET 200) parallel to each other, and closely spaced with respect to each other. Trenches 210 each has a width W, the first height H₁, and is spaced apart from adjacent trenches by a spacing S. For example, the spacing S between trenches 210 may be smaller than about 30 nm. The photo-sensitive layer 206 is then removed. Next, a cleaning may be performed to remove a native oxide of the semiconductor substrate 202. The cleaning may be performed using diluted hydrofluoric (DHF) acid.

In some embodiments, first height H₁ of the trenches 210 may range from about 2100 Å to about 2500 Å, while width W of the trenches 210 ranges from about 300 Å to about 1500Å. In an exemplary embodiment, the aspect ratio (H₁/W) of the trenches 210 is greater than about 7.0. In some other embodiments, the aspect ratio may even be greater than about 8.0. In yet some embodiments, the aspect ratio is lower than about 7.0 or between 7.0 and 8.0. One skilled in the art will realize, however, that the dimensions and values recited throughout the descriptions are merely examples, and may be changed to suit different scales of integrated circuits.

Liner oxide (not shown) is then optionally formed in the trenches 210. In an embodiment, liner oxide may be a thermal oxide having a thickness ranging from about 20 Å to about 500 Å. In some embodiments, liner oxide may be formed using in-situ steam generation (ISSG) and the like. The formation of liner oxide rounds corners of the trenches 210, which reduces electrical fields, and hence improves the performance of the resulting integrated circuit.

FIG. 4A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 4B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 4A. Trenches 210 are filled with a dielectric material 214. The dielectric material 214 may include silicon oxide, and hence is also referred to as oxide 214 in the present disclosure. In some embodiments, other dielectric materials, such as silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or a low-K dielectric material, may also be used. In an embodiment, the oxide 214 may be formed using a high-density-plasma (HDP) CVD process, using silane (SiH₄) and oxygen (O₂) as reacting precursors. In other embodiments, the oxide 214 may be formed using a sub-atmospheric CVD (SACVD) process or high aspect-ratio process (HARP), wherein process gases may comprise tetraethylorthosilicate (TEOS) and ozone (O₃). In yet other embodiment, the oxide 214 may be formed using a spin-on-dielectric (SOD) process, such as hydrogen silsesquioxane (HSQ) or methyl silsesquioxane (MSQ).

FIGS. 4A and 4B depict the resulting structure after the deposition of the dielectric material 214. A chemical mechanical polish is then performed, followed by the removal of the mask layer 204 b and pad layer 204 a. The resulting structure is shown in FIGS. 5A and 5B. FIG. 5A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 5B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 5A. The remaining portions of the oxide 214 in the trenches 210 are hereinafter referred to as insulation layers 216. In one embodiment, the mask layer 204 b is formed of silicon nitride, the mask layer 204 b may be removed using a wet process using hot H₃PO₄, while pad layer 204 a may be removed using diluted HF acid, if formed of silicon oxide. In some alternative embodiments, the removal of the mask layer 204 b and pad layer 204 a may be performed after the recessing of the insulation layers 216, which recessing step is shown in FIGS. 6A and 6B.

The CMP process and the removal of the mask layer 204 b and pad layer 204 a produce the structure shown in FIGS. 5A and 5B. FIG. 6A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 6B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 6A. As shown in FIGS. 6A and 6B, the insulation layers 216 are recessed by an etching step, resulting in recesses 218. In one embodiment, the etching step may be performed using a wet etching process, for example, by dipping the substrate 202 in hydrofluoric acid (HF). In another embodiment, the etching step may be performed using a dry etching process, for example, the dry etching process may be performed using CHF₃ or BF₃ as etching gases.

The remaining insulation layer 216 comprises a top surface 216 t extending upward from the substrate major surface 202 s to a second height H₂ less than the first height H₁, whereby upper portions 222 of the fins 212 extend beyond the top surface 216 t of the insulation layer 216. In one embodiment, a ratio of the second height H₂ to the first height H₁ is from about 0.5 to about 0.8. In the depicted embodiment, the upper portions 222 of the fins 212 may comprise channel portions 222 a and source/drain (S/D) portions 222 b. The channel portions 222 a are used to form channel regions of the FinFET 200. A third height H₃ of the upper portion 222 of the fins 212 may be between 15 nm and about 50 nm, although it may also be greater or smaller.

FIG. 7A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 7B is a cross-sectional view of a FinFET taken along the line a-a of FIG. 7A. A gate stack 220 is formed over the channel portions 222 a of the upper portion 222 of the fins 212 and extending to the top surface 216 t of the insulation layer 216. In some embodiments, the gate stack 220 typically comprises a gate dielectric layer 220 a and a gate electrode layer 220 b over the gate dielectric layer 220 a.

In FIGS. 7A and 7B, a gate dielectric 220 a is formed to cover the channel portions 222 c of the upper portion 222 of the fins 212. In some embodiments, the gate dielectric layer 220 a may include silicon oxide, silicon nitride, silicon oxynitride, or high-k dielectrics. High-k dielectrics comprise metal oxides. Examples of metal oxides used for high-k dielectrics include oxides of Li, Be, Mg, Ca, Sr, Sc, Y, Zr, Hf, Al, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and mixtures thereof. In the present embodiment, the gate dielectric layer 220 a is a high-k dielectric layer with a thickness in the range of about 10 to 30 angstroms. The gate dielectric layer 220 a may be formed using a suitable process such as atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal oxidation, UV-ozone oxidation, or combinations thereof. The gate dielectric layer 220 a may further comprise an interfacial layer (not shown) to reduce damage between the gate dielectric layer 220 a and channel portions 222 a of the upper portions 222 of the fins 212. The interfacial layer may comprise silicon oxide.

The gate electrode layer 220 b is then formed on the gate dielectric layer 220 a. In one embodiment, the gate electrode layer 220 b covers the upper portions 222 of more than one semiconductor fin 212, so that the resulting FinFET 200 comprises more than one fin. In some alternative embodiments, each of the upper portions 222 of the semiconductor fins 212 may be used to form a separate FinFET 200. In some embodiments, the gate electrode layer 220 b may comprise a single layer or multilayer structure. In the present embodiment, the gate electrode layer 220 b may comprise polysilicon. Further, the gate electrode layer 220 b may be doped polysilicon with the uniform or non-uniform doping. In some alternative embodiments, the gate electrode layer 220 b may include a metal such as Al, Cu, W, Ti, Ta, TiN, TiAl, TiAlN, TaN, NiSi, CoSi, other conductive materials with a work function compatible with the substrate material, or combinations thereof. In the present embodiment, the gate electrode layer 220 b comprises a thickness in the range of about 30 nm to about 60 nm. The gate electrode layer 220 b may be formed using a suitable process such as ALD, CVD, PVD, plating, or combinations thereof.

Still referring to FIG. 7A, the FinFET 200 further comprises a dielectric layer 224 formed over the substrate 202 and along the side of the gate stack 220. In some embodiments, the dielectric layer 224 may include silicon oxide, silicon nitride, silicon oxynitride, or other suitable material. The dielectric layer 224 may comprise a single layer or multilayer structure. A blanket layer of the dielectric layer 224 may be formed by CVD, PVD, ALD, or other suitable technique. Then, an anisotropic etching is performed on the dielectric layer 224 to form a pair of spacers on two sides of the gate stack 220. The dielectric layer 224 comprises a thickness ranging from about 5 to 15 nm.

FIG. 8A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 8B is a cross-sectional view of a FinFET taken along the line b-b of FIG. 8A. The structures depicted in FIGS. 8A and 8B are produced by selectively growing an epitaxial layer 230 covering the S/D portions 222 b of the upper portions 222 of the semiconductor fins 212, whereby the S/D portions 222 b are not covered by the gate stack 220 and the dielectric layer 224. Since the lattice constant of the epitaxial layer 230 is different from the substrate 202, the channel portions 222 a of the upper portion 222 of the fins 212 are strained or stressed to enable carrier mobility of the device and enhance the device performance.

Thermodynamically, growth rate of the close-packed (111) crystal plane of the epitaxial layer 230 is much greater than growth rates of other crystal planes of the epitaxial layer 230. The epitaxial layer 230 is therefore grown from the facets 230 a, 230 b, 230 c, 230 d, 230 e, 230 f, 230 g, and 230 h covering the S/D portions 222 b. In the depicted embodiment, the selective growth of the epitaxial layer 230 over each fin 212 continues until the epitaxial layer 230 vertically extends a distance above the S/D portions 222 b of the upper portions 222 of the fins 212 and laterally extends a space S₁ less than 1 nm from each other over the top surfaces 216 t of the insulation layer 216, thereby forming a cavity 240 between the adjacent epitaxial layers 230.

In the depicted embodiments, the epitaxial layer 230 may comprise a single layer or multilayer structure. In the single-layer embodiment, the epitaxial layer 230 may comprise a silicon-containing material. In some embodiments, the silicon-containing material comprises SiP, SiC, or SiGe. In one embodiment, the epitaxial layer 230, such as silicon carbon (SiC), is epi-grown by a LPCVD process to form the S/D regions of the n-type FinFET. The LPCVD process is performed at a temperature of about 400° to 800° C. and under a pressure of about 1 to 200 Torr, using Si₃H₈ and SiH₃CH as reaction gases. In another embodiment, the epitaxial layer 230, such as silicon germanium (SiGe), is epi-grown by a LPCVD process to form the S/D regions of the p-type FinFET. The LPCVD process is performed at a temperature of about 400° to 800° C. and under a pressure of about 1 to 200 Torr, using SiH₄ and GeH₄ as reaction gases.

In the multilayer embodiment, the epitaxial layer 230 may further comprise a II-VI semiconductor material or a III-V semiconductor material between the silicon-containing material and the S/D portions 222 b of the upper portions 222 of the semiconductor fins 212. In some embodiments, the II-VI semiconductor material comprises a material selected from the group consisting of ZeSe, ZnO, CdTe, and ZnS. In some embodiments, the III-V semiconductor material comprises a material selected from the group consisting of GaAs, InAs, InGaAs, AlAs, AlGaAs, InP, AlInP, InGaP, GaN, AlGaN, InN, InGaN, InSb, InGaAsSb, InGaAsN, and InGaAsP. In the depicted embodiment, the epitaxial layer 230, such as gallium arsenide (GaAs), is epi-grown by a metal-organic chemical vapor deposition (MOCVD) process. The MOCVD process is performed at a temperature of about 400° C. to 500° C., using trimethylgallium (TMGa) and monogerman (GeH₄) as reaction gases.

The process steps up to this point have provided the substrate 202 having the epitaxial layer 230 over each fin 212 laterally extending a space S₁ less than 1 nm from each other over the top surfaces 216 t of the insulation layer 216. Conventionally, silicide regions over the epitaxial layer 230 may be formed by blanket depositing a thin layer of metal material, such as nickel, titanium, cobalt, and combinations thereof. The substrate 202 is then heated, which causes silicon to react with the metal where contacted. After the reaction, a layer of metal silicide is formed between the silicon-containing material and the metal. The un-reacted metal is selectively removed through the use of an etchant that attacks the metal material but does not attack silicide. However, the small space between the adjacent epitaxial layers 230 may impede metal material from entering into the cavity 240, resulting in silicide formation in an upper portion of the epitaxial layers 230 but no silicide formation in a bottom portion of the epitaxial layers 230, This non-uniform distribution of silicide on epitaxial layers 230 (i.e., strained materials) causes high contact resistance of S/D regions of the FinFET and thus degrades the device performance.

Accordingly, the processing discussed below with reference to FIGS. 9A-9B may remove at least a lateral portion of the epitaxial layers 230 to enlarge the space between the adjacent epitaxial layers 230 to make it easier for metal depositions into the cavity 240. This can help silicide formation in the bottom portion of the epitaxial layers 230, thereby fabricating low contact resistance of S/D regions of the FinFET 200 and thus upgrading device performance.

FIG. 9A is a perspective view of the FinFET 200 at one of the various stages of fabrication according to an embodiment, and FIG. 9B is a cross-sectional view of a FinFET taken along the line b-b of FIG. 9A. The structures depicted in FIGS. 9A and 9B are produced by annealing the substrate 202 to have each fin 212 covered by a bulbous epitaxial layer 232 defining an hourglass shaped cavity 250 between adjacent fins 212, the cavity 250 comprising upper and lower portions 250 a, 250 b. In the depicted embodiments, facets 230 a, 230 b, 230 c, 230 d, 230 e, 230 f, 230 g, and 230 h of the epitaxial layer 230 are rounded to form the bulbous epitaxial layer 232. The bulbous epitaxial layer 232 thus laterally extends a space S₂ greater than the space S₁ from each other over the top surfaces 216 t of the insulation layer 216. The greater space S₂ make it easier for metal depositions into the hourglass shaped cavity 250. In one embodiment, a minimum width (i.e., the space S₂) of the hourglass shaped cavity is from about 3 nm to about 10 nm. In another embodiment, a ratio of a minimum width (i.e., the space S₂) of the hourglass shaped cavity 250 to a maximum width S₃ of the hourglass shaped cavity 250 is from about 0.1 to about 0.5.

In some embodiments, the step of annealing the substrate 202 to have each fin 212 covered by a bulbous epitaxial layer 232 is performed at a temperature between about 800° C. to 1100° C. and under a pressure of about 5 Torr to 760 Torr and a flow rate of about 5 sccm to 200 sccm, using H₂ or D₂ as a reaction gas. In alternative embodiments, the step of annealing the substrate 202 to have each fin 212 covered by a bulbous epitaxial layer 232 may further comprise flowing a carrier gas over the substrate 202. In some embodiments, the carrier gas comprises N₂, He, or Ar.

FIGS. 10-14B also are cross-sectional views of the FinFET 200 taken along the line b-b of FIG. 9A at one of the various stages of fabrication according to an embodiment. Referring to FIG. 10, after formation of the bulbous epitaxial layer 232, a first metal material 260 is formed over the bulbous epitaxial layer 232 to a thickness of between about 15 and 60 angstroms. In the depicted embodiment, the first metal material 260 comprises a material selected from the group consisting of titanium, cobalt, nickel, platinum, erbium, and palladium. The first metal material 260 may be formed by CVD, PVD, plating, ALD, or other suitable technique.

Then, the structures depicted in FIGS. 11A-12B are produced by annealing the substrate 202 to convert the bulbous epitaxial layer 232 bordering the lower portion 250 b of the cavity 250 to silicide 262. In other words, the first metal material 260 in contact with the bulbous epitaxial layer 232 is then transformed into the silicide 262 by a thermal process, such as a rapid thermal anneal (RTA) process. In one embodiment, the silicide 262 is conformal if the bulbous epitaxial layer 232 is partially transformed into the silicide 262. In another embodiment, the silicide 262 is uniform if the bulbous epitaxial layer 232 is fully transformed into the silicide 262. In the single-layer embodiment, the silicide 262 may be conformal or uniform. In the multilayer embodiment, the silicide 262 is conformal and over the II-VI semiconductor material or III-V semiconductor material.

In the conformal embodiment, the structures depicted in FIGS. 11A and 11B are produced by annealing the substrate 202 to convert the bulbous epitaxial layer 232 bordering the lower portion 250 b of the cavity 250 to silicide 262, wherein the bulbous epitaxial layer 232 bordering the upper portion 250 a of the cavity 250 is converted to silicide 262 thicker than the silicide 262 bordering the lower portion 250 b of the cavity 250. In some embodiments, the silicide 262 comprises a material selected from the group consisting of titanium silicide, cobalt silicide, nickel silicide, platinum silicide, erbium silicide, and palladium silicide.

In the conformal embodiment, a first RTA process is applied to the substrate 202 at a temperature between about 200° C. and 300° C. and for between about 10 and 20 seconds. The first metal material 260 in contact with the bulbous epitaxial layer 232 will form a high-resistance silicide. Then, the remaining un-reacted first metal material 260 may be etched away using, for example, a solution comprising NH₄OH, H₂O₂, and deionized water. In order to transform the high-resistance silicide to a low-resistance silicide 262, a second RTA process is applied to the substrate 202 at a temperature between about 300° C. and 500° C. and for between about 10 and 30 seconds (shown in FIG. 11A). In at least one alternative embodiment, if the remaining un-reacted first metal material 260 is not fully etched away, the lower portion 250 b of the cavity 250 comprises the first metal material 260 below the silicide 260 bordering the lower portion 250 b of the cavity 250 (shown in FIG. 11B).

In the uniform embodiment, the structures depicted in FIGS. 12A and 12B are produced by annealing the substrate 202 to convert the bulbous epitaxial layer 232 bordering the lower portion 250 b of the cavity 250 to silicide 262, wherein the bulbous epitaxial layer 232 bordering the lower portion 250 b of the cavity 250 is fully converted to silicide 262. In some embodiments, the silicide 262 comprises a material selected from the group consisting of titanium silicide, cobalt silicide, nickel silicide, platinum silicide, erbium silicide, and palladium silicide.

In the uniform embodiment, a first RTA process is applied to the substrate 202 at a temperature between about 200° C. and 300° C. and for between about 10 and 30 seconds. The first metal material 260 in contact with the bulbous epitaxial layer 232 will form a high-resistance silicide. Then, the remaining un-reacted first metal material 260 may be etched away using, for example, a solution comprising NH₄OH, H₂O₂, and deionized water. In order to transform the high-resistance silicide to a low-resistance silicide 262, a second RTA process is applied to the substrate 202 at a temperature between about 300° C. and 500° C. and for between about 30 and 60 seconds (shown in FIG. 12A). In alternative embodiment, if the remaining un-reacted first metal material 260 is not fully etched away, the lower portion 250 b of the cavity 250 comprises the first metal material 260 below the silicide 262 bordering the lower portion 250 b of the cavity 250 (shown in FIG. 12B).

FIGS. 13A, 13B, 14A, and 14B show the FinFETs 200 of FIGS. 11A, 11B, 12A, and 12B after depositing a second metal material 270 over the silicide 262 and top surface 216 t of the insulation layer 216 to fill the upper portion 250 a and lower portion 250 b of the cavity 250, i.e., the upper and lower portions 250 a, 250 b of the cavity 250 comprises the second metal material 270. In the depicted embodiment, the second metal material 270 comprises Al, Cu, or W. In some embodiments, the second metal material 270 may be formed by CVD, PVD, ALD, or other suitable technique.

It is understood that the FinFET 200 may undergo further CMOS processes to form various features such as contacts/vias, interconnect metal layers, dielectric layers, passivation layers, etc. Thus, Applicants' method can help silicide formation in the bottom portion of the epitaxial layers, thereby fabricating low contact resistance of S/D regions of the FinFET 200 and thus upgrading device performance.

In one embodiment, a fin field effect transistor (FinFET) comprises a substrate comprising a major surface; a first fin and a second fin extending upward from the substrate major surface to a first height; an insulation layer comprising a top surface extending upward from the substrate major surface to a second height less than the first height, whereby portions of the fins extend beyond the top surface of the insulation layer; each fin covered by a bulbous epitaxial layer defining an hourglass shaped cavity between adjacent fins, the cavity comprising upper and lower portions, wherein the epitaxial layer bordering the lower portion of the cavity is converted to silicide.

In another embodiment, a method for fabricating a fin field effect transistor (FinFET) comprises providing a substrate comprising a major surface; forming a first fin and a second fin extending upward from the substrate major surface to a first height; forming an insulation layer comprising a top surface extending upward from the substrate major surface to a second height less than the first height, whereby portions of the fins extend beyond the top surface of the insulation layer; selectively growing an epitaxial layer covering each fin; annealing the substrate to have each fin covered by a bulbous epitaxial layer defining an hourglass shaped cavity between adjacent fins, the cavity comprising upper and lower portions; forming a metal material over the bulbous epitaxial layer; and annealing the substrate to convert the bulbous epitaxial layer bordering the lower portion of the cavity to silicide.

While the disclosure has been described by way of example and in terms of specific embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A fin field effect transistor (FinFET) comprising: a substrate comprising a major surface; a first fin and a second fin extending upward from the substrate major surface to a first height; and an insulation layer comprising a top surface extending upward from the substrate major surface to a second height less than the first height, whereby portions of the fins extend beyond the top surface of the insulation layer, wherein each fin is covered by a bulbous epitaxial layer defining an hourglass shaped cavity between adjacent fins, the cavity comprising upper and lower portions, and wherein the bulbous epitaxial layer bordering the lower portion of the cavity is converted to silicide.
 2. The FinFET of claim 1, wherein a ratio of the second height to the first height is from about 0.5 to about 0.8.
 3. The FinFET of claim 1, wherein a minimum width of the hourglass shaped cavity is from about 3 nm to about 10nm.
 4. The FinFET of claim 1, wherein a ratio of a minimum width of the hourglass shaped cavity to a maximum width of the hourglass shaped cavity is from about 0.1 to about 0.5.
 5. The FinFET of claim 1, wherein the bulbous epitaxial layer bordering the lower portion of the cavity is fully converted to silicide.
 6. The FinFET of claim 1, wherein the bulbous epitaxial layer bordering the upper portion of the cavity is converted to silicide thicker than the silicide bordering the lower portion of the cavity.
 7. The FinFET of claim 1, wherein the bulbous epitaxial layer comprises SiP, SiC, or SiGe.
 8. The FinFET of claim 1, wherein the bulbous epitaxial layer comprises a II-VI semiconductor material or a III-V semiconductor material.
 9. The FinFET of claim 1, wherein the silicide comprises a material selected from the group consisting of titanium silicide, cobalt silicide, nickel silicide, platinum silicide, erbium silicide, and palladium silicide.
 10. The FinFET of claim 1, wherein the lower portion of the cavity comprises a first metal material below the silicide bordering the lower portion of the cavity.
 11. The FinFET of claim 10, wherein the first metal material comprises a material selected from the group consisting of titanium, cobalt, nickel, platinum, erbium, and palladium.
 12. The FinFET of claim 1, wherein the upper and lower portions of the cavity comprise a second metal material.
 13. The FinFET of claim 12, wherein the second metal material comprises Al, Cu, or W.
 14. A fin field effect transistor (FinFET) comprising: a substrate comprising a major surface; a first fin and a second fin extending upward from the substrate major surface; an insulation material on the major surface between the first fin and the second fin; a bulbous epitaxial layer over each of the first fin and the second fin, the bulbous epitaxial layer defines an hourglass shaped cavity between the first fin and the second fin; and a first metal material over a top surface of the insulation material in the hourglass shaped cavity.
 15. The FinFET of claim 14, further comprising a silicide layer surrounding the bulbous epitaxial layer.
 16. The FinFET of claim 14, wherein the bulbous epitaxial layer is a silicide material.
 17. The FinFET of claim 14, further comprising a gate structure over the first fin and the second fin, wherein the bulbous epitaxial layer over the first fin and the second fin are positioned adjacent to the gate structure.
 18. The FinFET of claim 14, wherein a ratio of a minimum width of the hourglass shaped cavity to a maximum width of the hourglass shaped cavity is from about 0.1 to about 0.5.
 19. The FinFET of claim 14, wherein the bulbous epitaxial layer is a multilayer structure.
 20. The FinFET of claim 14, further comprising a second metal material surrounding the bulbous epitaxial layer and the first metal material. 